// Adler32.cs - Computes Adler32 data checksum of a data stream
// Copyright (C) 2001 Mike Krueger
//
// This file was translated from java, it was part of the GNU Classpath
// Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License
// as published by the Free Software Foundation; either version 2
// of the License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
//
// Linking this library statically or dynamically with other modules is
// making a combined work based on this library.  Thus, the terms and
// conditions of the GNU General Public License cover the whole
// combination.
// 
// As a special exception, the copyright holders of this library give you
// permission to link this library with independent modules to produce an
// executable, regardless of the license terms of these independent
// modules, and to copy and distribute the resulting executable under
// terms of your choice, provided that you also meet, for each linked
// independent module, the terms and conditions of the license of that
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// or based on this library.  If you modify this library, you may extend
// this exception to your version of the library, but you are not
// obligated to do so.  If you do not wish to do so, delete this
// exception statement from your version.

using System;

namespace ICSharpCode.SharpZipLib.Checksums
{
    /// <summary>
    ///     Computes Adler32 checksum for a stream of data. An Adler32
    ///     checksum is not as reliable as a CRC32 checksum, but a lot faster to
    ///     compute.
    ///     The specification for Adler32 may be found in RFC 1950.
    ///     ZLIB Compressed Data Format Specification version 3.3)
    ///     From that document:
    ///     "ADLER32 (Adler-32 checksum)
    ///     This contains a checksum value of the uncompressed data
    ///     (excluding any dictionary data) computed according to Adler-32
    ///     algorithm. This algorithm is a 32-bit extension and improvement
    ///     of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
    ///     standard.
    ///     Adler-32 is composed of two sums accumulated per byte: s1 is
    ///     the sum of all bytes, s2 is the sum of all s1 values. Both sums
    ///     are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
    ///     Adler-32 checksum is stored as s2*65536 + s1 in most-
    ///     significant-byte first (network) order."
    ///     "8.2. The Adler-32 algorithm
    ///     The Adler-32 algorithm is much faster than the CRC32 algorithm yet
    ///     still provides an extremely low probability of undetected errors.
    ///     The modulo on unsigned long accumulators can be delayed for 5552
    ///     bytes, so the modulo operation time is negligible.  If the bytes
    ///     are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
    ///     and order sensitive, unlike the first sum, which is just a
    ///     checksum.  That 65521 is prime is important to avoid a possible
    ///     large class of two-byte errors that leave the check unchanged.
    ///     (The Fletcher checksum uses 255, which is not prime and which also
    ///     makes the Fletcher check insensitive to single byte changes 0 -
    ///     255.)
    ///     The sum s1 is initialized to 1 instead of zero to make the length
    ///     of the sequence part of s2, so that the length does not have to be
    ///     checked separately. (Any sequence of zeroes has a Fletcher
    ///     checksum of zero.)"
    /// </summary>
    /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream" />
    /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream" />
    public sealed class Adler32 : IChecksum
    {
        /// <summary>
        ///     largest prime smaller than 65536
        /// </summary>
        private const uint BASE = 65521;

        /// <summary>
        ///     Creates a new instance of the Adler32 class.
        ///     The checksum starts off with a value of 1.
        /// </summary>
        public Adler32()
        {
            Reset();
        }

        #region IChecksum Members

        /// <summary>
        ///     Returns the Adler32 data checksum computed so far.
        /// </summary>
        public long Value
        {
            get { return checksum; }
        }

        /// <summary>
        ///     Resets the Adler32 checksum to the initial value.
        /// </summary>
        public void Reset()
        {
            checksum = 1;
        }

        /// <summary>
        ///     Updates the checksum with a byte value.
        /// </summary>
        /// <param name="value">
        ///     The data value to add. The high byte of the int is ignored.
        /// </param>
        public void Update(int value)
        {
            // We could make a length 1 byte array and call update again, but I
            // would rather not have that overhead
            uint s1 = checksum & 0xFFFF;
            uint s2 = checksum >> 16;

            s1 = (s1 + ((uint) value & 0xFF))%BASE;
            s2 = (s1 + s2)%BASE;

            checksum = (s2 << 16) + s1;
        }

        /// <summary>
        ///     Updates the checksum with an array of bytes.
        /// </summary>
        /// <param name="buffer">
        ///     The source of the data to update with.
        /// </param>
        public void Update(byte[] buffer)
        {
            if (buffer == null)
            {
                throw new ArgumentNullException("buffer");
            }

            Update(buffer, 0, buffer.Length);
        }

        /// <summary>
        ///     Updates the checksum with the bytes taken from the array.
        /// </summary>
        /// <param name="buffer">
        ///     an array of bytes
        /// </param>
        /// <param name="offset">
        ///     the start of the data used for this update
        /// </param>
        /// <param name="count">
        ///     the number of bytes to use for this update
        /// </param>
        public void Update(byte[] buffer, int offset, int count)
        {
            if (buffer == null)
            {
                throw new ArgumentNullException("buffer");
            }

            if (offset < 0)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("offset");
#else
                throw new ArgumentOutOfRangeException("offset", "cannot be negative");
#endif
            }

            if (count < 0)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("count");
#else
                throw new ArgumentOutOfRangeException("count", "cannot be negative");
#endif
            }

            if (offset >= buffer.Length)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("offset");
#else
                throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");
#endif
            }

            if (offset + count > buffer.Length)
            {
#if NETCF_1_0
				throw new ArgumentOutOfRangeException("count");
#else
                throw new ArgumentOutOfRangeException("count", "exceeds buffer size");
#endif
            }

            //(By Per Bothner)
            uint s1 = checksum & 0xFFFF;
            uint s2 = checksum >> 16;

            while (count > 0)
            {
                // We can defer the modulo operation:
                // s1 maximally grows from 65521 to 65521 + 255 * 3800
                // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
                int n = 3800;
                if (n > count)
                {
                    n = count;
                }
                count -= n;
                while (--n >= 0)
                {
                    s1 = s1 + (uint) (buffer[offset++] & 0xff);
                    s2 = s2 + s1;
                }
                s1 %= BASE;
                s2 %= BASE;
            }

            checksum = (s2 << 16) | s1;
        }

        #endregion

        #region Instance Fields

        private uint checksum;

        #endregion
    }
}